The invention relates to methods and apparatus for reducing the distortion appearing in an amplified output signal. In particular, the invention relates to reducing distortion by predistorting an input signal.
It is known to predistort the input signal to an amplifier to counter distortion imposed on the input signal by the amplifier. This makes the amplifier output assume a more linear relationship to the input signal. There are several known predistortion techniques for countering intermodulation distortion (IMD) imposed upon a signal by an amplifier.
One such predistortion technique comprises removing a sample of the input signal (prior to amplification) and mixing it into itself repeatedly to produce components whose frequencies correspond to the frequencies of intermodulation distortion appearing in the output of the amplifier. These components are injected into the input signal to predistort it in such a way as to counter the intermodulation distortion imposed on it by the amplifier. It will be appreciated that the components produced by the predistortion signal by mixing the input signal sample into itself repeatedly only approximately mimic the IMD produced by the amplifier. Hence, IMD cancellation by this predistortion technique is imperfect.
A second known technique for predistorting the input to an amplifier comprises the input signal through it. The output of the low power amplifier should therefore contain components whose frequencies correspond to the IMD created by the main amplifier operating on the main component of the input signal. These components can then be injected into the input signal to the main amplifier as a controllable predistortion. However, it will be appreciated that, again, the predistortion signal only approximately mimics the IMD characteristics of the main amplifier because the low power amplifier constructed will not have distortion characteristics which match perfectly those of the main amplifier.
It is an object of the invention to provide a predistortion which accurately mimics the distortion characteristics of an amplifier which it is desired to linearise.
It is another object of the invention to provide a predistortion in a power-efficient manner. According to a first aspect, the invention provides a method of reducing the distortion appearing in an amplified output signal, the method comprising the steps of: sampling a portion of an input signal destined for amplification by a first amplification process; amplifying the sample signal using a second amplification process whose distortion characteristics are at least similar to those of the first amplification process to produce an amplified sample signal; extracting a distortion signal from the amplified sample signal; combining the distortion signal with the input signal destined for amplification by the first amplification process to reduce distortion of its output; and combining the output of the first and second amplification processes to produce the amplified output signal; wherein the step of extracting a distortion signal comprises the steps of sampling the amplified sample signal and removing the amplified input signal appearing in this sample.
According to a second aspect, the invention provides apparatus for reducing the distortion appearing in an amplified output signal, the apparatus comprising means for sampling a portion of the input signal destined for amplification by a first amplification means, second amplification means for amplifying the sample signal, to produce an amplified sample signal, the distortion characteristics of the second amplification means being at least similar to those of the first amplification means, means for extracting a distortion signal from the amplified sample signal, means for combining the distortion signal with the input signal destined for amplification by the first amplification means to reduce distortion of its output, and means for combining the outputs of the first and second amplification means to produce the amplified output signal; wherein the extracting means comprises means for sampling the amplified sample signal and means for removing the amplified input signal appearing in this sample.
The invention thus provides that the power present in the output of the second amplification process is not wasted but forms part of the output signal, thereby enhancing efficiency. Furthermore, this makes it practical for the second amplification process to be a relatively high output power process, like the first (main) amplification process, which means that the distortion characteristics of the two amplification processes can match one another more accurately.
In a preferred embodiment, the first and second amplification processes are implemented by amplifiers which are identical to one another. In a variation on this embodiment, the main amplifier comprises a number of amplification modules, which are identical to one another, operating in parallel on the input signal and the second amplification process (which creates the distortion signal for the predistortion process) comprises a smaller number of amplification modules of the aforementioned type operating in parallel. The arrangements used in these embodiments provide that the distortion characteristics of the two amplification processes match very well.
The extracted distortion signal is subjected to amplitude and phase adjustment to ensure maximum cancellation of distortion in the output signal of the first (and, possibly, also in the second) amplification process. This allows the system to be adjusted as the distortion characteristics of the amplification processes vary over time, due, for example, to ageing and/or temperature variations.
In a preferred embodiment, the invention provides for the input signal to the first (main) amplification process to be predistorted in such a way as to overcorrect the output signal of the first amplification process in a controlled manner. The benefit of doing this is that the controlled distortion in the output of the first amplification process can be arranged to cancel distortion present in the output of the second amplification process with which it is combined, thereby leading to a more linear overall output signal.